High-Yield Alpha-Cellulose from Oil Palm Empty Fruit Bunches by Optimizing Thermochemical Delignification Processes for Use as Microcrystalline Cellulose

Oil palm empty fruit bunches (OPEFB) are lignocellulosic materials that are a by-product of the palm oil industry, which have less use and utilization is still limited. OPEFB's high cellulose content could potentially develop into various bioproducts, especially biomaterials. The thermochemical delignification process can obtain high-yieldalpha-cellulose. The cellulose extraction process can be done by combining the bleaching process under acidic conditions and alkaline delignification to obtain high-purity cellulose. The bleaching conditions vary in the concentration of NaClO2, the length of bleaching, the temperature, and the number of stages. The research obtains high α-cellulose by optimizing bleaching conditions under acidic conditions in cellulose's OPEFB extraction with variability on NaClO2 concentration and bleaching time using response surface methodology (RSM). The bleaching process was implemented at an early stage with a concentration of 3% NaClO2 and a bleaching time of 2 hours as a center point with a bleaching cycle of twice at pH 4–4.5 using acetic acid. Bleached fibers were delignified using 10% NaOH for 2 hours at room temperature. The RSM analysis resulted in optimum bleaching conditions at a concentration of 3.22% NaClO2 for 1 hour, yielding OPEFB's cellulose of 82.96% ± 2.53, hemicellulose of 9.27% ± 2.28, and lignin of 1.68% ± 0.58. The validation and verification process in the bleaching conditions obtained cellulose of 84.87% and α-cellulose of 88.51%, with a crystallinity index of 70.55% and crystallite size of 2.35 nm. Scanning electron microscopy on surface cellulose morphology at optimum bleaching helped remove hemicellulose impurities, lignin, and inorganic materials and a more intensive opening of cellulose fibrils. The bleaching process optimization point was verified to improve the delignification performance and potentially produce high yield α-cellulose content for microcrystalline cellulose use.


Introduction
Indonesia is the largest oil palm producer in the world and has the largest plantation. In 2021, Indonesia will have an area of 15 million hectares with a production of 49.71 million tons [1]. Te potential for oil palm empty fruit bunches (OPEFB) waste is 25% (12.42 million tons) and utilized only 10%. OPEFB waste potential is 15 percent or 11.18 million tons [2]. OPEFB is relatively abundant [3] and has benefts as an alternative energy source [4,5], fertilizers, chemical components, and biomaterials [6].
OPEFB, as the biomaterial source, needs proper cellulose isolation technology to obtain high purity so it can be optimized using cellulose as its derivative product. Te potential market for microcrystalline cellulose products is quite large in the pharmaceutical, cosmetic, and food industries. Microcrystalline cellulose is widely used as a binder [17][18][19][20], fller [21][22][23][24][25][26], and absorbent [27,28]. OPEFB cellulose, as a raw material for microcrystalline cellulose, requires high levels of alpha cellulose, which afects its crystallinity. OPEFB has a complex and waxy structure; therefore, an efcient extraction process is required. Chemical treatment is the most efective way to obtain higher-purity cellulose. Combining bleaching with NaClO 2 , alkali treatment, and acid hydrolysis is commonly used for extracting cellulose [29][30][31][32].
Chemical methods using alkaline and acid solutions help remove lignin and some hemicelluloses and reduce the degree of polymerization. Tis technique requires a low cost to remove acetyl groups and lignin at low pressure and low temperature and increases the crystallinity of cellulose [16,33,34].
Several methods have been studied. Removing hemicellulose and lignin fractions in physical hydrothermal pretreatment was carried out at a temperature of 150°C to 200°C at a high pressure of about 30 bar with a reaction time of 5-25 minutes [35]. Tis method is environmentally friendly but requires high energy. Pretreatment using an ionic solution such as bmimHSO 4 (1-butyl-3-methylimidazolium hydrogen sulfate) has a high solvation but is expensive [36]. Ultrasound-H 2 O 2 techniques at low temperatures and shorter times are not optimal in cellulose isolation [37]. Meanwhile, steam explosion treatment at 160-260°C in a short time can reduce lignin and hemicellulose content but can cause excessive degradation of cellulose. Te enzymatic delignifcation method is environmentally friendly but results in slow delignifcation [38].
In the long term, the combination of sodium chlorite and alkaline pretreatment of OPEFB is an economical method for pulp treatment due to the application of low operating temperatures and pressures [33,34]. Te preparation conditions, such as the type of chemicals used, the concentration used, and the duration and temperature of the hydrolysis treatment, infuenced the yield of cellulose extracted from the fber. Previous studies performed cellulose extraction from plants using alkaline treatment, followed by NaClO 2 bleaching [39][40][41][42][43]. Alkali treatment was only able to partially remove hemicellulose and lignin. Chemical treatment, such as acidic bleaching, can improve high cellulose content. Terefore, the bleaching process obtains the desired purity of cellulose.
Bleaching before alkaline delignifcation using 0.7% NaClO 2 for fve cycles resulted in 81.1% cellulose [15]. Pujiasih et al. [14] prepared OPEFB cellulose to produce MCC with a crystallinity index of 66.99% through bleaching of 15% NaClO 2 for three cycles and a long OPEFB cellulose preparation step to fnd extracted cellulose for CMC production [44]. Based on preliminary research results, delignifcation using 17.5% NaOH at room temperature and bleaching under acidic conditions using NaClO 2 3% twice produced cellulose greater than 80%. Under acidic conditions, the bleaching process not only plays a role in whitening the fber but also helps extract cellulose more optimally.
Chemical processes efectively produce high-purity cellulose. Te delignifcation process cannot work alone but requires a combination of bleaching under acidic conditions. Te delignifcation process with NaOH and KOH was only partially able to remove lignin and hemicellulose. Te isolation process was complemented by a bleaching process with NaClO 2 under acidic conditions to remove lignin, hemicellulose, and partial depolymerization.
An efcient process requires a low temperature, a low bleacher concentration, and a short bleaching cycle, thereby shortening the production process. Optimizing the bleaching process under acidic conditions to help remove lignin and hemicellulose has not been investigated. Te acidic conditions in the bleaching process are adjusted using acetic acid, which is environmentally friendly. Te acidic conditions for bleaching are carried out after reaching 70°C so that NaClO 2 has reacted efectively. In previous studies, the acidic conditions of bleaching were not clearly explained. Terefore, this research aimed to optimize the bleaching under acidic conditions, especially to get the optimum bleacher concentration and bleaching time for process effciency and enhance delignifcation that produces highcontentα-cellulose. Obtaining optimization of bleacher concentration, minimum cycle, and minimum processing time with a moderate process will be very benefcial for the industrialization of α-cellulose from OPEFB in the oil palm industry. Te conversion of OPEFB waste into derivative products can improve its added value.

Experimental Design from Response Surface Methodology.
Te primary data on the chemical quality of cellulose were collected from each treatment, which included water extractives, hemicellulose, cellulose, and lignin. Te treatment was designed from an optimization process carried out using the Response Surface Methodology (RSM) with two factors, namely, the bleaching time with a center point of 2 hours and NaClO 2 concentration with a center point of 3%, thus obtaining 13 experimental units with 5 points as repetitions at the center. Te design used for the optimization analysis 2 International Journal of Biomaterials uses a central composite with a quadratic target model; the details of the factors are in Table 1 and the experimental design is carried out according to Table 2.

OPEFB Preparation.
OPEFB was washed with hot water, and the fbers were separated manually. Fibers were rinsed for up to quartz washings using clean water. Furthermore, the fber was soaked in a 2% soap solution (ratio of fber to soap � 1 : 4) for 5 hours to remove residual oil and dust. Te remaining soap (and other dirts) was rinsed with clean water twice. Te washed fbers were then drained and dried in an oven at 60°C for 48 hours. Te clean and dry OPEFB was cut into ±5 cm, ground, and then sieved to obtain a size of 30 mesh.

Cellulose Extraction Process (Bleaching and
Delignifcation). Ten grams of OPEFB fber was bleached using NaClO 2 according to the treatment of NaClO 2 concentration, with a fber-to-NaClO 2 solution ratio1 : 25 (w/v). Te solution was heated until the temperature of 75°C ± 5°. Next, the bleacher solution was added acetic acid to adjust the pH to 4-4.5 and heated constantly at a temperature of 75°C ± 5°with bleaching time according to the treatment variation. Acidifcation by glacial acetic acid at pH 4-4.5 was done twice. After the frst bleaching, the solution was fltered and continued in the second cycle at the same bleacher concentration and bleaching time. Te bleached fbers were washed, dried, and weighed. Te delignifcation of bleached fbers used a 10% NaOH solution with a ratio of 1 : 20 w/v at room temperature. Te delignifed cellulose was washed with hot, distilled water. Te cellulose was refuxed for 30 minutes with distilled water for washing and then dried at 60°C ± 0.5°for 24 hours.

Characterization of OPEFB Fiber and Extracted
OPEFB Cellulose. Cellulose was tested, including waterextractive material, hemicellulose, cellulose, and lignin. Tese parameters were used to optimize the process conditions. Te chemical structure of cellulose was analyzed by attenuated total refectance-Fourier transform infrared spectroscopy (ATR-FTIR) (Bruker 200546 Model Alpha), the crystallinity analysis was carried out by X-ray difraction (X-RD) (Rigaku MiniFlex Hypix-400MF 2D HPAD detector),and the morphology analysis of OPEFB cellulose was carried out using scanning electron microscopy (SEM; Brand FEI, Inspect-S50 type).

Test Method for Fiber and Cellulose Components.
Fiber components were analyzed using the Chesson method [45]. One gram of powder was added to 150 mL of distilled water and refuxed for 2 hours. Te sample was fltered and washed until the pH was neutral, put in an oven at 105°C to dry, then weighed and calculated using equation (1). Residue 1 was added to 150 mL of 0.5 M H 2 SO 4 and refuxed for 2 hours. Te sample was fltered and washed until the pH was neutral, put in an oven at 105°C to dry, then weighed and calculated using equation (2). Residue 2 was added to 10 mL of 72% H 2 SO 4 and macerated for 4 hours at room temperature. Te sample was added to 150 mL of 0.5 M H 2 SO 4 and refuxed for 2 hours. Te sample was fltered and washed until the pH was neutral, put in an oven at 105°C to dry, then weighed and calculated using equation (3). Te calculation of lignin content using equation (4) is as follows: water extractive material content(%) � initial mass − mass of residue 1 initial mass hemicellulose content(%) � mass of residue 1 − mass of residue 2 initial mass × 100%, where I 002 indicates the maximum intensity of the 002 peaks around 2θ � 22.0°-23.0°and I am is the lowest intensity corresponding to the value of 2θ around 15.0°-17.0°2 .8. Data Analysis. Water extractive materials, hemicellulose, cellulose, and lignin data were analyzed using Response Surface Methodology with Design Expert version 12 software From StateEase, Minneapolis, to determine the optimization point on treatment factors. RSM tests the ft of the model regression (lack of ft), the regression parameters simultaneously, and the residual assumption that the residual must meet the normal assumption. Furthermore, a response surface analysis was carried out to obtain the optimum point. Te data on the crystallinity index, crystallite size, and morphology of the cellulose structure were presented descriptively.
To obtain cellulose from OPEFB fber, bleaching and delignifcation were combined. Extraction optimization was applied to the bleaching process of OPEFB using NaClO 2 at pH 4-4.5 with two cycles of bleaching and delignifcation using 10% NaOH once. Optimal conditions are as follows: center point concentration of 3% and the length of a bleaching process is 2 hours.
Te bleaching process tends to identify the decolorization process. However, the bleaching conditions at an acidic pH will help open the structure and partially depolymerize it, and reducing hemicellulose and lignin will be easier. Te decrease in hemicellulose and lignin will continue at the base delignifcation stage.
Te content of hemicellulose, cellulose, and lignin varies depending on the NaClO 2 concentration and bleaching time. At the same bleaching cycle, which is twice, the concentration of NaClO 2 and bleaching time will afect the cellulose and lignin content. A NaClO 2 concentration of less than 3% is insufcient to produce high cellulose purity (>80%). Table 3 presents data on the content of water extractive material, hemicellulose, cellulose, holocellulose, and lignin in extracted cellulose from OPEFB.
Te cellulose obtained from the treatment varied depending on the concentration of NaClO 2 and the bleaching time applied. A concentration of 3% NaClO 2 with a bleaching time of 0.59 hours obtained the highest cellulose content. Lignin residues were still quite signifcant in the cellulose obtained in the C1.5T1, C0.88T2, and C1.5T3 treatments. During the same bleaching cycle, low NaClO 2 concentrations are insufcient to reduce lignin in OPEFB fbers signifcantly. Te NaClO 2 concentration range of 3% to 5.12% signifcantly reduced lignin, and the residual lignin was 1-2.83%.
Bleaching under acidic conditions helps open the OPEFB cell wall structure, especially the outer layer, in the form of a wax layer. Bleaching using NaClO 2 under acidic conditions has been used for cellulose isolation [14,15,44,48,49]. Research by Soetaredjo et al. [50] produced 77.8% cellulose from OPEFB with a combination of delignifcation of NaOH 2 N at 6 hours and microwave. Septevani et al. [8], using 10% NaOH delignifcation at 150°C pressure for 4 bar for 30 minutes and bleaching NaClO 2, were able to obtain 84.3% cellulose purity, and Yimlamai et al. [51] obtained 83.7% cellulose using peracetic acid in two stages and combinations of H 2 O 2 and NaOH in the delignifcation process.
Te intensive depolymerization process at the initial acid bleaching stage helps reduce the cellulose's impurities, hemicellulose, and lignin components. According to Mussatto et al. [52], acid pretreatment will disintegrate the fber to facilitate delignifcation. Base components will quickly enter the structure, and the degradation of lignin is more efcient so that the release of cellulose occurs. Firstly, hydrolyzation quickly occurs in hemicellulose, causing hemicellulose to bind to cellulose with hydrogen bonds. Lignin binds to cellulose with covalent bonds on the inside of the cell structure, so that the acidic conditions in bleaching not only remove color components (chromophores) but also help remove lignin.

Determination of Model Optimization.
Te extraction of cellulose is optimized from OPEFB to obtain the optimum concentration of NaClO 2 and bleaching time, which is capable of getting high purity of cellulose. Optimization was carried out on the water-extractive material, hemicellulose, cellulose, and lignin contents. Tere are several optimization parameters to consider sequentially, including ft summary, lack-of-ft tests, analysis of variance (ANOVA), and analysis of diagnostic plots for model validation, followed by analysis of multiple response optimization using graphical and numerical tools. Table 4 presents a model that fts each parameter.
Te ft summary showed the feasibility of the formed model from each response or parameter for water, hemicellulose, cellulose, and lignin extractive materials. Te water-extractive material response was linear, with hemicellulose in 2F1 and cellulose and lignin in a quadratic model. Te results of the ANOVA analysis for the response of water extractives and hemicellulose showed that the independent variables of bleaching time and NaClO 2 concentration did not signifcantly afect the model (p value >0.05). Otherwise, the variable concentration of NaClO 2 and the square of the concentration of NaClO 2 signifcantly afect the model (p value <0.05) for cellulose and lignin responses.
Te parameters of cellulose and lignin produced a quadratic model. In contrast, the air extractive material parameters were linear, and hemicellulose models were produced in the 2F1 model (between linear and quadratic). Te lack of ft indicates the model's acceptance of the model, which is not signifcant (p value >0.05). Table 5 shows the lack of ft value was insignifcant for all parameters (p value >0.05), namely, the air extractive material parameter with a p value of 0.9948, a p value of hemicellulose 0.0998, a p value of cellulose 0.3721, and a p value for lignin 0.2024. Lack of conformity is a deviation or inaccuracy to the model, with an insignifcant p value indicating the model is acceptable, and the error does not afect the model signifcantly.
Te normality of the data must support the signifcance of the model. Data analysis showed that plotting residual data on the parameters of the extractive material water, hemicellulose, cellulose, and lignin is normal. Te scattering of the data indicates a normal distribution following a straight line. Figure 1 describes the normality data for all optimization parameters.
Te model predicted a relationship with the response between the bleaching time (X 1 ) and NaClO 2 (X 2 ) concentration. Te signifcance model in optimization is described by the quadratic model. Te responses of cellulose and lignin resulted in the quadratic model. Te following is the equation of the model on the parameters of the water extractives (Ye), hemicellulose (Yh), cellulose (Ys), and lignin (Yg): Te quadratic equation on the cellulose response (Ys) showed a positive correlation between the single factor of bleaching time and NaClO 2 concentration, their interaction, and the square of the bleaching time factor (increases cellulose). However, the square of the concentration of NaClO 2 has a negative correlation with the cellulose produced. In the quadratic model of the response of cellulose and lignin, the single-factor NaClO 2 concentration and the International Journal of Biomaterials 5 squared concentration of NaClO 2 had a signifcant efect on the model. In the quadratic equation of lignin response, the square of NaClO 2 concentration positively correlated to lignin. However, the single factor of NaClO 2 concentration, the interaction, and the square of the bleaching time factor had a negative efect (reduced) on the lignin content. Figure 2 shows the contour and surface model responses for each response.

Solution of Optimization.
Te optimal solution point for bleaching time and NaClO 2 concentration was determined using the target parameter value approach. Te constraints data to fnd the optimized point are presented in Table 6. Optimization conditions are justifed in the water extractive material, hemicellulose, and lignin content at a minimum, while cellulose is at a maximum. Furthermore, all parameters were set at the same level of importance: level 3. Te optimized bleaching process conditions determined by RSM resulted in optimized bleaching at a NaClO 2 concentration of 3.22% and a bleaching time of 1 hour. Figure 3 describes the validation of the optimization conditions. A quadratic model directs the optimization point on cellulose and lignin with maximum and minimum cellulose content.

Optimization of Point Confrmation.
Te RSM found the optimization point of the bleaching process at a NaClO 2 concentration of 3.22% and a bleaching time of 1 hour, and then the confrmation step was carried out. On the confrmed response surface at the optimization point, the cellulose content obtained was 82.96% ± 2.53 with a 95% confdence level. Table 7 presents the complete data on confrmation from optimized bleaching. OPEFB cellulose obtained from the optimization point is higher than the standard cellulose content of 80.80% [8].
Confrmation and verifcation of optimization conditions were done by carrying out the bleaching process at 3.22% NaClO 2 and a 1 hour bleaching time. After delignifcation, a cellulose content of 84.87%, α-cellulose 88.51%, hemicellulose of 9.80%, holocellulose of 94.67%, and water extractive material of 3.36%. Te optimization point is verifed to produce an optimal extraction process. Optimized bleaching conditions in acidic conditions before  International Journal of Biomaterials delignifcation can improve the performance of delignifcation and produce high-purity cellulose. According to Septevani et al. [8], the alkaline delignifcation process combined with bleaching NaClO 2 provides high selectivity, reducing hemicellulose and lignin simultaneously without damaging the cellulose structure. Te average content of α-cellulose in the C3T2 and C3.22T1, respectively, was 84.52% and 88.51%. α-Cellulose in C3.22T1 treatment range from 83.02% to 94.00%. α-Cellulose is insoluble cellulose at a concentration of 17.5% NaOH. In this study, high α-cellulose is essential to produce microcrystalline cellulose (MCC) as raw material for hydrogel fller and enhance the hydrogel's mechanical strength. α-Cellulose has a high degree of polymerization. MCC is produced from partially depolymerized α-cellulose by hydrolysis of excess mineral acid [17]. MCC is characterized by a high degree of crystallinity, the value of which is usually in the range of 55% to 80% [53].
Kim [54] stated that cellulose is the main constituent of lignocellulosic biomass. Te cellulose content of lignocellulosic biomass varies from 30 to 50%. Cellulose molecules combine as microfbers, in which highly ordered (crystalline) regions alternate with less regular ones (amorphous). Moreover, cellulose strongly tends to form intra and intermolecular hydrogen bonds. Hemicellulose has a less stable structure; that is, it is more amorphous than cellulose and consequently more easily hydrolyzed by acids into monomers.
Acid pretreatment conditions easily degrade into decomposition products, including furfural. Te lignin molecules are cross-linked and have high molecular weights, ranging from 12% to 33% by weight in lignocellulosic biomass. Lignin functions in plants to unite cellulose fbers and provide strength to lignocellulosic biocomposites [55].

Fourier Transmittance Infrared Spectroscopy.
FTIR analysis showed the consistency of the functional groups in cellulose. Bleaching treatment with various concentrations of NaClO 2 and bleaching time will provide several changes, namely, a decrease in hemicellulose and lignin and an increase in the amount of cellulose that varies. Te results of the FTIR analysis provide a qualitative description of the wave numbers on the structure of hemicellulose, cellulose, and lignin.     [56]. Figure 5 shows that the treatment of C3T0.59 and C5.12T2 sharpened the wave numbers in the range of 3300 cm −1 , 2890 cm −1 , and 1016 cm −1 . Te wavenumber is the area of cellulose and holocellulose. Shifting wavenumber indicated increased cellulose purity and decreased hemicellulose and lignin impurities. Popescu et al. [56] stated that changes in wavenumber could indicate cellulose structure degradation by acid-bleaching, and cellulose might be eroded and defbrillated.
Identifcation of wave numbers in the treatment C0.88T2, C3T0.59, and C5.12T2 as follows: at wavenumbers, 3300 cm −1 to 2900 cm −1 is a distinct area of O-H and C-H bonds in polysaccharides. Te broad peak at 3320 cm −1 is the hydroxyl stretching vibration and the cellulose inter and intramolecular hydrogen bond vibrations. Typical wavenumbers indicated cellulose range from 1670 cm −1 to 900 cm −1 . Te absorbance peak of 1016 cm −1 correlates with the vibrations of the water molecules absorbed in the cellulose [57]. Bands at absorption 1649 cm −1 , 1319 cm −1 , and 1016 cm −1 produced stretching and bending vibrations of -CH2 and -CH, -OH, and C-O bonds in cellulose [58]. According to Zhang et al. [59], the chloric acid solution has been successfully used to break the ether bonds between lignin and cellulose; this is evidenced by the loss of aromatic skeletal vibrations in lignin at wavenumbers 1.505 cm −1 and 1.592 cm −1 , and the ester bond between hydroxyl lignin and carboxyl uronic acid in hemicellulose is disrupted during alkaline treatment [16].

X-Ray
Difraction. An X-ray difraction test showed changes in the crystallinity of cellulose by treatment with variations in NaClO 2 concentration and bleaching time. Te crystallinity index (CI) parameter describes cellulose's relative amount of crystalline material. Te two-phase cellulose model describes the cellulose chains as containing crystalline and amorphous regions. XRD analysis found cellulose peak intensity at 2θ of 22°-23°and 2θ at the amorphous intensity at 15°-17°. Figure 6 shows the peak sharpening in the C3T2 and C3T0.59 treatments compared to EFB fber. In Table 8, the crystallinities of C3T2 and C3T0.59 were 85.78% and 86.56%, respectively. Te highest crystallinity index in the C5.12T2 treatment was 89.15%. Te cellulose crystallinity is higher than that of EFB fber. Bleaching under acidic conditions and alkaline delignifcation can reduce hemicellulose and lignin, which have an amorphous structure. Kim [54] stated that the crystallite structure is related to the reduced amorphous structure.
Bleaching under acidic conditions followed by delignifcation degrades the amorphous cellulose structure, thereby increasing the degree of crystallinity. High crystallinity is very important in microcrystalline cellulose, especially in its use as a fller that can strengthen the mechanical structure, such as in tablets and hydrogel flms. Table 8 provides an overview of the consistent pattern of the efect of NaClO 2 concentration and bleaching time. However, it does not fully describe a linear relationship where a higher NaClO 2 concentration and bleaching time will increase the crystallinity index. Te C4.5T3 treatment has a crystallinity index of 72.64%. Tis is in line with previous research. EFB fbers were bleached using 15% Na hypochlorous acid (1 : 25 w/v) for 2 hours at 80°C three times, followed by delignifcation using 17.5% NaOH (1 : 12.5 w/v) for 2 hours at room temperature twice to produce cellulose crystallinity of 66.99% [14]. Cellulose crystallinity is associated with tensile strength, which is essential to consider the cellulose used as its derivative product.
One of the causes of peak widening is the presence of an amorphous structure. However, on the other hand, intrinsic factors that afect peak widening include crystal size and nonuniform strain in the crystal. Te peak in OPEFB is broader than in cellulose. Te hemicellulose content is still high in OPEFB. Peak cellulose is due to amorphous cellulose. However, crystal size is equally important for peak broadening, and several studies have assumed that crystal size is a signifcant contributor [60]. Te width of the crystal peak (002) at half height is directly related to the crystal size, and the crystallite size's cellulose is about 4 to 7 nm in most references [61].
Te crystallite size of cellulose-based OPEFB ranges from 1.36 to 3.42 nm, and cellulose has a small crystallite size. Crystalline cellulose is imperfect; thus, most cellulose structures are less regular and amorphous. Changes in crystallite size have not been fully correlated to the bleaching factor under acidic conditions. According to Popescu et al. [56], the cellulose structure is more complicated than indicated by the two-phase (crystalline and amorphous) model. Te amount of paracrystalline cellulose (33.1%) was almost identical to the amount of crystal structure (31.8%) in cotton cellulose. Te existence of a transition region between the crystalline and amorphous structures makes it more challenging to interpret the crystallinity of cellulose. Likewise, if the amorphous structure is closed inside the crystallite structure, it will be difcult to react with the amorphous component, so changes in crystallinity will be challenging to predict.

Scanning Electron Microscopy.
Morphological changes appear on the surface where, in OPEFB fber, the surface morphology still looks rough; compared to fbers that have undergone bleaching, the surface area is smoother. Morphological changes in the extracted cellulose were smoother in the C3T2, C3.22T1, and C4.5T3 treatments. Tis smoother surface correlated with cellulose's physical properties, which were softer than the fber after bleaching.
Te bleaching process causes this subtle morphological change under acidic conditions, and alkaline delignifcation reduces the number of impurities, especially hemicellulose and lignin. Hemicellulose and lignin are amorphous, but hemicellulose is hydrogen bonded to cellulose, so it will be easier to remove than covalently bonded lignin.
Te morphological diferences of cellulose in the C1.5T3, C3T2, and C4.5T3 treatments indicated that the C3T2 and C4.5T3 treatments had been able to open the cellulose structure. Figure 7 shows the stretching of the structure's fbrils, which can later afect the degree of crystallinity of cellulose. Cellulose at optimum conditions C3.22T1 showed a morphology similar to C3T2. Te C4.5T3 treatment showed a high concentration of NaClO 2 , and prolonged bleaching in acidic conditions had a more signifcant grinding efect on the cellulose structure; this would correlate with the lower yield amount in the treatment.
Based on Figure 7, the treatment of C3T2, C3.22T1, and C4.5T3 resulted in an increasingly intensive fbril separation. Tere was decreased wax, hemicellulose, and lignin structure; a smooth and scar-like surface supported this. According to Nazir et al. [63,64], a scar-like surface is due to the removal of inorganic materials such as silica left behind by carbon and oxygen.
Te study applied the whole fber of OPEFB, where the hard part at the end of the stalk was not separated and is  relatively more stubborn to pretreatment than the long fber of the EFB structure. Reneta Nafu et al. [65] showed that more extensive degradation and intensive fbril opening occurred in OPEFB stem fbers with a softer structure than stalk fbers.

Conclusions
Te bleaching process under acidic conditions is efective for improving the delignifcation performance to obtain high cellulose. Based on the response surface method, the results of the optimization of bleaching under acidic conditions using NaClO 2 at pH 4-4.5 obtained the optimum bleaching conditions at a concentration of 3.22% NaClO 2 for 1 hour and continued with the delignifcation of NaOH. Under these conditions, the purity of cellulose was 82.96% ± 2.53, hemicellulose 9.27% ± 2.28, and lignin 1.68% ± 0.58. High α-cellulose was needed in MCC production as a mechanical strength function. α-Cellulose in C3.22T1 treatment ranged from 83.02% to 94.00%. XRD test results showed that under these conditions, the samples had a crystallinity index of 70.55% and a crystallite size of 2.35 nm. Based on the SEM test, the morphology on the surface of the cellulose showed that the bleaching treatment under acidic conditions helped remove hemicellulose impurities, lignin, and inorganic materials, as well as a more intensive opening of cellulose fbrils. Cellulose from OPEFB has great potential in terms of quality and quantity, so the oil palm industry needs to use it in an integrated manner to become a high-value industrial product. Te oil palm industry not only dumps OPEFB waste onto land as fertilizer but also converts OPEFB cellulose into microcrystalline cellulose, which is useful as a fller for various products or other cellulose derivative products.

Data Availability
Te data used to support the fndings of this study are included within this article, and necessary data can be obtained from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Conceptualization was done by SS and MA. Methodology was provided by SS, MA, WW, and MAF. Project administration was handled by SS and MA.